Recent Submissions

  • From Farming to Fishing: Marine Resource Conservation

    Versleijen, Nicole; Hoorweg, Jan (Western Indian Ocean Marine Science Association, 2008)
    This paper examines the arrival of a new group of fishermen on the Kenyan coast and what this has meant for the state of fishery resources. It reviews four subject areas: access and the number of fishermen; the fishermen’s identity; the choice of fishing gear; and the fishing grounds selected. Data were collected from a small number of fishing households in the villages of Uyombo and Takaungu in Kilifi District, using mainly qualitative research methods. Local households on the Kenyan coast face increasing pressure on land as well as on marine resources. The declining economic situation and greater pressure on land have made people turn to fishing as an income-generating activity. This group of fishermen is referred to as the ‘new’ generation of fishermen as they have been involved in fishing for only one or two generations (including the current one) in contrast to the ‘old’ generation from families who have been fishing or in fishingrelated activities for much longer. The old generation of fishermen and their households have also diversified their incomes, with many fishing households turning to farming, for example, with women and grown-up children involved in various activities. The new generation of fishermen, mainly of the Mijikenda population group, has often been blamed for the loss of traditional access regulations and for using harmful fishing gear. This paper discusses the new generation of fishermen and their identity as they perceive it and relates this to employment generation as a policy measure for marine conservation.
  • Coral reefs and their management in Tanzania

    Wagner, G.M. (Western Indian Ocean Marine Science Association, 2004-12-30)
    Coral reefs are very important in Tanzania, both ecologically and socio-economically, as major fishing grounds and tourist attractions. Numerous fringing and patch reefs are located along about two-thirds of Tanzania’s coastline. These reefs have been partially to severely degraded by human (primarily destructive fishing practices) and natural (particularly coral bleaching) causes. These immediate human causes have been brought about by various socioeconomic root causes, particularly poverty and lack of proper management. After decades of human and natural impacts there has been only limited reef recovery. This paper presents a region-by-region analysis of trends in the condition of coral reefs in Tanzania in relation to the causes of damage. While earlier approaches to management were aimed at non-use of coral reefs in marine protected areas (seldom achieved), recent approaches have aimed at integrated coastal management (ICM) (whether in programs or conservation areas), where zonation into core protected areas and multiple-use areas is based on participatory decision-making involving fishing communities and other stakeholders. Some management initiatives also involve communities in reef monitoring, restoration and ecotourism. This paper examines the management approaches and strategies implemented by various ICM programs, conservation areas and marine parks in Tanzania. It also provides recommendations for further research and coral reef management strategies.
  • Spatial and temporal distribution of reef fish spawning aggregations in the Seychelles – an interview-based survey of artisanal fishers

    Öhman, M.C.; Isidore, M.; Payet, R.J.; Robinson, J.; Marguerite, M.A. (WIOMSA, 2004)
    Many coral reef fish species aggregate at specific times and locations for the purpose of spawning. This study examined the spatial and temporal distribution of spawning aggregations in the Seychelles. An interview-based survey of the principal stakeholders, mainly artisanal fishers, yielded 89 reports of aggregation fishing for 26 demersal and semi-pelagic fish species. Grouper aggregations were largely concentrated in the outer coralline islands of the exclusive economic zone, whilst those of snappers and rabbitfishes were mainly reported from locations on the Seychelles Bank. The spatial patterns among fish families were attributed to a combination of differences in species abundance and distribution as well as variation in fishing effort. Spawning periodicity reported by fishers indicated that for snappers and rabbitfishes, the most activity occurred across a protracted period of October to April/May, with peaks in activity at either end of that period. Grouper spawning activity was concentrated in the northeast monsoon months of November to January. The findings of this study suggest that several spawning aggregations are targeted by fishers on a regular basis, a practice that constitutes a primary issue for artisanal fisheries research and management in the Seychelles.
  • Digestive endo-proteases from the midgut glands of the Indian white shrimp, Penaeus Indicus (Decapoda: Penaeidae) from Kenya.

    Omondi, J.G. (WIOMSA, 2005)
    In order to provide information on the digestive capacity of marine crustacea of mariculture potential in Kenya with a view to aiding the development of suitable feeds to support the industry, a biochemical survey was made of enzymes of extracellular digestion in the Indian White shrimp, Penaeus indicus. Results showed midgut gland endo-proteases in wild adult shrimp from the Kenya coast to have optima between pH 7.2 and 8.5 (Trypsin pH 7.5-8.0, Chymotrypsin pH 7.2-7.8, Elastase pH 6.8-8.5) with maximum specific activities of 101-408, 37-516, 70-90 Units mg protein-1 min-1 for trypsin, chymotrypsin and elastase respectively. There was no pepsin. The North Sea Norway lobster, Nephrops norvegicus, was investigated to a lesser extent and found to have much lower trypsin activity than the shrimp and no chymotrypsin. In addition to the cited serine endo-proteases, significant activity in the shrimp was thought to originate from non-serine proteases. This situation may differ from other shrimps in which serine endo-protease activity, especially trypsin, is dominant. Diphenylcarbamyl chloride (DPCC) and 2-Nitro-4-Carboxyphenyl N,N-Diphenylcarbamate (NCDC) inhibited chymotrypsin but not trypsin, Soybean Trypsin Inhibitor (SBTI), Bowman-Birk Chymotrypsin-Trypsin Inhibitor (BBSTCI), N-Tosyl-L-Phenylalanine Chloromethyl Ketone (TPCK), 4-(2-Aminoethyl)-Benzenesulfonylfluoride Hydrochloride (AEBSF) and N-Tosyl-Llysine Chloromethyl Ketone/1-Chloro-3-Tosylamido-7-Amino-L-2-Heptanone Hydrochloride (TLCK) inhibited both, while Phenyl Methanesulfonyl Fluoride/ Phenylmethyl Sulfonyl Fluoride/ á-Toluenesulfonyl Fluoride (PMSF) and Ovomucoid Trypsin Inhibitor (Ovomucoid/OTI) precipitated shrimp homogenate. The effect of the former was inferred from the action of AEBSF which together with TLCK inhibited shrimp trypsin more than chymotrypsin. In contrast, TPCK inhibited shrimp chymotrypsin more than trypsin. These results indicate that relying on imported commercial feeds, usually developed for other species or strains of farmed shrimp in other parts of the world, may not only be too uneconomical but may not provide adequate nutrition to local animals if not efficiently digested. There is, therefore, greater need and urgency to establish detailed enzymic profiles and digestive capacities of locally cultured fin and shellfish. Such studies should parallel those prospecting for suitable feed ingredients while developing local capacity for feed technology.
  • Characterisation of chitosan from blowfly larvae and some crustacean species from Kenyan marine waters prepared under different conditions.

    Oduor-Odote, P.M.; Struszczyk, M.H.; Peter, M.G. (WIOMSA, 2005)
    Isolation of chitosan from cuticles of blue bottlefly larvae Calliphora erythrocephala, and shells of crab Sylla cerrata, lobster Panulirus ornatus, prawn Paeneaus indicus was carried out. The yield of chitin was 12.0%, 23.0%, 15.7% and 28.0% respectively. In the same order the yield of chitosan was 66.0%, 74.6% 74.3% and 75.0% from chitin. Ash in the crab and lobster chitosan demineralised with 0.5M HCl was 30.2 and 22.4% respectively. This was reduced to 0.2 % for lobster and 0.4% for crab using 2M HCl for demineralisation and 0.5M HCL was adequate for demineralisation of prawns to bring the ash content to < 1%. The ash content in the blowfly larvae was negligible. The conditions used for chitosan isolation in blowfly larvae were milder requiring no demineralisation step. The time to obtain soluble chitosan in 1% v/v acetic acid was 8 hr for crab and lobster at 100°C deacetylation and 4 hr at 120°C while for prawns it was 6 hr at 100°C and 3 hr at 120°C deacetylation temperature. The average molecular weight (¯MV) for crabs was 556,000 after 8 hr deacetylation and 148,000 at 140°C deacetylation temperature. With 2M HCl used for demineralisation first, it was 439,000 for a 4 hr period. Crabs, first demineralised then deprotenised the ¯MV was 155,000 for a 3 hr deacetylation at 120°C and 417,000 for 1 hr deacetylation. An 8 hr deacetylation at 100°C for lobsters gave ¯MV of 791,000. It was reduced to 560,000 after 4 hr of deacetylation at 120°C and to 236,000 at 140°C for 3 hr. Prawns had a ¯MV of 507,000 after 6 hr deacetylation at 100°C and reduced to 455,000 after a 3 hr deacetylation. For insect larvae, at 100°C deacetylation for 4 hr the ¯MV was 413,500 while for 1 hr, 2 hr and 2.5 hr deacetylation time at 120°C it was 369,000, 308,500 and 263,000 respectively. The degree of deacetylation (DD) increased with temperature and time of deacetylation. For crab, demineralised then deproteinised, it increased from 72.9% in 1 hr then 81.5% in 3 hr. In prawn chitosan it was 60.0% for the 6 hr deacetylation at 100°C and 69.2% for 3 hr deacetylation at 120°C. The DD of insect larvae was 62.56% after 4 hr of deacetylation at 100°C. When deacetylated at 120°C it was 64.0% after 1 hr, 79.9% after 2 hr and 80.7% after 2.5 hr. The moisture content showed a slight increase with DD. Temperature increase and time of deacetylation caused a decrease in MV and a more conservative increase in DD.
  • An antimalarial cembranolide from Tanzania soft corals, Lobophytum crassum (von Marenzeller (1886) and L. rotundum (Tixier-Durivault (1957).

    Said, S.A. (WIOMSA, 2005)
    Bioscreening guided fractionation of the extracts of soft corals Lobophytum crassum and L. rotundum using brine shrimp larvae cytotoxicity assay led to the isolation of a cembranolide diterpene (E,E,E)-6,10,14-trimethyl-3-methylene- 3a,4,7,8,11,12,15,15aoctahydrocylotetradeca [b]furan-2(3H)-one (1). This diterpene, identified as cembranolide compound 1, was found to be active against the multidrug-resistant and chloroquine-sensitive strains of Plasmodium falciparum malaria parasite in vitro. It also possessed cytotoxic properties.
  • Monitoring shoreline change using remote sensing and GIS: a case study of Kunduchi area, Tanzania

    Makota, V.; Sallema, R.; Mahika, C. (WIOMSA, 2004)
    Data from aerial photographs taken in 1981, 1992 and 2002 of the Kunduchi shoreline off the Dar es Salaam coast were integrated in a geographic information system (GIS) to determine shoreline change in that locality. It was found that considerable changes have taken place, and that the two techniques are effective for shoreline monitoring.
  • Cyanobacterial occurrence and diversity in seagrass meadows in coastal Tanzania

    Lyimo, T.J.; Hamisi, M.I.; Muruke, M.H.S. (WIOMSA, 2004)
    We report on the occurrence and diversity of cyanobacteria in intertidal seagrass meadows at Ocean Road and Mjimwema, Dar es Salaam, Tanzania. Nutrients, temperature and salinity were measured as comparative environmental factors. A total of 19 different cyanobacteria taxa were encountered, out of which eight were found exclusively in Mjimwema, four exclusively in Ocean Road and seven were common to both sites. Oscillatoria, Lyngbya and Spirulina were the dominant cyanobacterial genera. Cyanobacterial coverage was higher in Mjimwema (31–100%) than in Ocean Road (0–60%). The levels of nutrients in tidal pool waters at Ocean Road ranged from 0.45–1.03 ìmol NO3 -N/l, 0.19–0.27 ìmol NO2 -N/l and 0.03–0.09 ìmol PO4 -P/l. At Mjimwema the nutrient concentration ranges were 0.14–0.93 ìmol NO3 -N/l, 0.20–0.30 ìmol NO2 -N/l and 0.01-0.07 ìmol PO4 -P/l . The nutrient levels were significantly higher at Ocean Road than at Mjimwema (P = 0.001 for nitrate and P = 0.025 for phosphate). There was no significant difference in nitrite levels between the study sites (P = 0.83). The low cyanobacterial diversity and coverage in Ocean Road is related to the high levels of nutrients and physical disturbance from sewage discharge and the harbour in the area.
  • The occurrence and distribution of dolphins in Zanzibar, Tanzania, with comments on the differences between two species of tursiops

    Amir, O.A.; Jiddawi, N.S.; Berggren, P. (WIOMSA, 2005)
    Incidental catches (bycatch) in gillnet fisheries off Zanzibar (Unguja Island), as a source of mortality among several species of dolphins, were reported in a questionnaire survey conducted in 1999. As a follow-up to that survey, from January 2000 to August 2003, we monitored the incidental catches of dolphins collected from 12 fish landing sites. Six species of dolphins were recorded from 143 specimens retrieved from bycatches in drift- and bottom set gillnets. Of these, 68 (48%) were Indo-Pacific bottlenose dolphins (Tursiops aduncus), 44 (31%) spinner dolphins (Stenella longirostris), 12 (8%) Risso’s dolphins (Grampus griseus), 11 (8%) Indo-Pacific humpback dolphins (Sousa chinensis), 6 (4%) Pan-tropical spotted dolphins (Stenella attenuata) and 2 (1%) common bottlenose dolphins (Tursiops truncatus). Most of the bycatches (71%) were in nets set off the north coast of Unguja Island. In this paper, bycatch records are examined to describe the occurrence and distribution of dolphin species in Unguja Island coastal waters. The relatively large numbers of bycatch dolphins recorded indicate that bycatch may be a potential threat to local populations that need to be addressed in future conservation and management efforts in the region.
  • Fecundity and population structure of cockles, Anadara antiquata L. 1758 (Bivalvia: Arcidae) from a sandy/muddy beach near Dar es salaam, Tanzania.

    Mzighani, S. (WIOMSA, 2005)
    The fecundity and population structure of the cockle, Anadara antiquata L., from sandy/muddy intertidal habitats in the intertidal of Ocean Road beach was examined from January to December 2001. A. antiquata is hand harvested, for domestic consumption, mainly by women and children, with the empty shells sold to traders. The sex ratio from combined data of all classes deviates significantly from 1:1 with the ratio of females to males increasing significantly above 41 mm shell length. Hermaphrodites were also observed in this size range indicating a protrandric hermaphrodite situation with males maturing earlier than females. Fecundity increases with an increase in shell length, with a mean of 1,652,000 (+/- 562,000 [or 40%] SE) eggs per female and correlated significantly with shell length and whole live weight. Two peaks were observed from length frequency analysis, one dominated by juveniles and other with mature A. antiquata. The slope b from length-weight relationship for A. antiquata was 2.7134 (n = 1,951) indicating isometric growth.
  • Reproductive cycle of edible echinoderms from the South-Western Indian Ocean: II. The sandfish Holothuria scabra (Jaëger, 1833)

    Vaïtilingon, D.; Rasolofonirina, R.; Eeckhaut, I.; Jangoux, M. (WIOMSA, 2005)
    The reproductive cycle and gonad morphology were investigated in a population of Holothuria scabra (Jaëger, 1833) from Toliara, south-west of Madagascar. Surveys were done from November 1998 to April 2001 by monthly samplings of 30 individuals. There is a single gonad formed by a tuft of numerous ramified tubules. The annual reproductive cycle of the population was determined from monthly variations of the gonad index and the maturity index. Five sexual maturity stages were described. An annual reproductive cycle occurred with most individuals being mature or spawning between November and April. Gametogenesis was not synchronous in the population, but was synchronous in all the tubules of a single gonad except the smallest immature ones (length < 1 cm). The length of tubules is not uniform in a gonad, and the number of gonad tubules is related to the size of the animal. Each tubule had a clear annual cycle, being the smallest in recovery gonads (regressed tubules) and the largest at the end of the maturation period (developed tubules). Recruitment of new tubules appears to occur throughout the year. Observations made suggest that the newly recruited tubules remain immature up to the beginning of the next reproductive cycle.
  • Reproductive cycle of edible echinoderms from the Southwestern Indian Ocean: I. Tripneustes gratilla L. (Echinoidea, Echinodermatata)

    Vaïtilingon, D.; Rasolofonirina, R.; Jangoux, M. (WIOMSA, 2005)
    The reproductive cycle of the edible echinoid Tripneustes gratilla (Linneaus 1758) was investigated in a population located in the southwest coast of Madagascar (Toliara). The reproductive cycle was followed for two consecutive years (October 1999 to October 2001) and characterised by means of gonad and maturity indices. T. gratilla has an annual reproductive cycle mediated by seawater temperature, day length and feeding activity. The histology of the gonads revealed six different maturity stages grouped in three main reproductive phases: growing and maturing, spawning, and spent and recovering. Reproduction is seasonal. Growing and maturing stages occurred from late summer to early winter correlating with decreasing temperatures and day length. Spawning occurred during mid and later winter, when temperatures and day lengths were lowest. Spent and recovering urchins were observed from late summer to early winter. Besides, the reproductive cycle was also influenced by food quantity in the gut, the repletion index was higher after spawning and gametogenesis started when sufficient amount of nutrient reserves accumulated in gut tissues and nutritive phagocytes of the gonads. This study reveals that the maturity index, based on histological analysis of gonads, is more reliable than gonad index in characterising the reproductive cycle of echinoids. In view of fishery management, we recommend harvest of T. gratilla from November to February, when individuals are in the pre-gametogenic stages.
  • Distribution and community structure of butterflyfishes (Pisces: Chaetodontidae) in southern Mozambique.

    Pereira, M.A.M.; Videira, E.J.S. (WIOMSA, 2005)
    Even though Mozambique has the greatest reported diversity of butterflyfishes (24 species) of the continental states of the Western Indian Ocean region, aspects of the ecology and distribution of this group in Mozambique are poorly documented. The distribution, diversity and community structure of butterflyfishes were studied on nine reefs in southern Mozambique using the point count method. Nineteen species from four genera were identified. Three with generalist feeding habits (i.e. Chaetodon auriga, C. guttatissimus and C. interruptus) were the most abundant with a wide distribution range. The butterflyfish communities of intertidal reefs at Inhaca Island differed from those of the offshore, subtidal reefs, as shown by uni- and multivariate analysis of abundance and diversity data. This is attributed to differences in habitat structure and food availability. However, most species occurred on both reef types. The butterflyfish diversity of the area is considered high and comparable to other high latitude reef areas in the world. These results emphasize the high biodiversity of the region and constitutes a latitudinal biodiversity peak. These findings also highlight the need for effective conservation measures.
  • Comparative study of three transect methods to assess coral cover, richness and diversity

    Beenaerts, N.; Berghe, E.V. (WIOMSA, 2005)
    Three different transect methods were compared at two different sites in Kenya, on their efficiency to estimate hard coral cover, genus richness and Shannon diversity index. For a modified line transect method (LTM), the line intercept method (LIT) and a linear point intercept (LPI) method the relative efficiency of the three methods was calculated with respect to the tested parameters. The three methods were examined along identical transect lines (10 m) and a total of 27 transect triplets were recorded in Vipingo and 21 in Mombasa. The correlation coefficients for all three ecological parameters were calculated for the three possible pairs of methods, and the accumulation curves plotted for each of the parameters using number of transects as the independent variable. Results from the three methods were virtually indistinguishable. When the parameters were plotted using measuring time on the x axis, the curves for the LPI method converged twice as fast as those for LTM, while LIT time was intermediate. It is suggested that the LPI method might be most suitable for assessing coral cover, richness and diversity where time and effort are significant constraint
  • A preliminary investigation of age and growth of Otolithes ruber from KwaZulu-Natal, South Africa

    Fennessy, S.T.; Brash, J.M. (WIOMSA, 2005)
    This study investigated age and growth of Otolithes ruber, found in KwaZulu-Natal, South Africa. The specimens were collected from prawn trawlers that operate off the shallow water Tugela Bank and from a recreational boat fishery in Durban. Estimates of age and growth parameters were based on the examination of sectioned sagittal otoliths. There was difficulty in estimating growth parameters for separate sexes because the small fish were not sexed and the numbers of males was low. Periodicity of growth zone formation was assumed to be annual although periodicity of growth zone deposition could not be established by marginal zone analysis. The von Bertalanffy growth curve was used to describe the combined male and female growth of O. ruber: Lt = 419mm TL (1-e-0.31 (t+0.96)) The maximum age estimated was eight years. The repeatability of the age estimates was relatively high (Average Percentage Error: 12.5%). The information gathered from this study will be used in a subsequent stock assessment.
  • Preliminary investigations on the ichthyodiversity of Kilifi Creek, Kenya.

    Oyugi, D. (WIOMSA, 2005)
    In order to determine fish spatial variation, the Kilifi Creek was divided into three sampling zones: entrance, middle zone, and Creek end. Sampling was by both gill and cast nets. Hill’s diversity indices were used as a measure of spatial diversity variance. A total of 95 species from 45 families were recorded. Gerres filamentosus, Pomadasys multimaculatum, Leiognathus equula, and Terapon theraps occurred commonly. Highest species richness (No = 68) and diversity (N1 = 31.09) was recorded at the entrance and Creek middle respectively. Most of the species overlap with the highest overlap index (L = 2.52) recorded between Cynoglossus gilchristi and Scombroides lysan. The creek’s ichthyodiversity show considerable variability, with the high richness at the entrance attributed to the presence of open water visitor species. The high number of species compares well to records from Gazi Bay and Mida Creek with some species not found further south.
  • Population genetic status of the Western Indian Ocean: what do we know?

    Sampayo, E.M.; Ridgway, T. (WIOMSA, 2005)
    Population genetics offers a useful technique for studying the population structure of marine organisms and has relevance to both systematics and the conservation of biodiversity. The Western Indian Ocean (WIO) is faced with increasing evidence of degradation and effective management initiatives are needed to curtail the environmental decline. The management of the WIO region can therefore benefit from the information that population genetics can provide. Extensive literature searches revealed only 31 genetic references for the WIO region. From a biogeographic point of view, the WIO shows little genetic exchange with the rest of the Indo- Pacific, but from a regional perspective, the limited information that exists points towards widespread genetic structuring in the reefs off tropical Africa and the Indian Ocean islands, and greater connectivity amongst southeast African reefs. However, much more information is needed in the region before the true strength of population genetic data can be used as a primary tool for management.
  • Participatory mapping of terrestrial fishery resources in Kwale District, Kenya

    Kimani, P.; Obura, D. (WIOMSA, 2004)
    A participatory mapping study was conducted in Kinondo location of Kwale district in Kenya. The area is subject to high human population and development pressure in the north, where tourism and urban development is intense, compared to lower population pressure in the south where such development is low. Our focus was to document information about these resources that have maintained fishing for many generations and to address issues of their conservation and management. The study focused on the spatial arrangement, access, ownership and use of land-based resources, mainly shrubs, grasses and trees, at four landing sites along the north-south population gradient. Participatory techniques (sketch maps, livelihood diagrams and transect walks) were applied, where trained fishers led other fishers in mapping, and the local Digo language was used for recording names. Resources were arranged in distinct vegetation zones parallel to the beach. A north-south increase in resource availability and abundance was recorded, inversely related to the higher population pressure in the north compared to the south. While much of the land on which vegetation resources are located was publicly accessible, a significant part is owned by absentee landlords, concentrated in the north. This over time results in increasingly restricted access to resources by fishermen, as shown in the more developed, northern section of the study site. Fishers also experience problems with access routes to landing sites on the beach due to encroachment on routes by beachfront development. The mapping activity revealed the potential for conflict over resource access and the need for solutions to maintain fishers’ access to terrestrial resources important for fishing.
  • Juvenile penaeid shrimp density, spatial distribution and size composition in four adjacent habitats within a Mangrove-Fringed Bay on Inhaca Island, Mozambique.

    Macia, A. (WIOMSA, 2004)
    The effects of habitat characteristics (mangrove creek, sandflat, mudflat and seagrass meadow) water salinity, temperature, and depth on the density, spatial distribution and size distribution of juveniles of five commercially important penaied shrimp species (Metapenaus monoceros, M. stebbingi, Fenneropenaeus indicus, Penaeus japonicus and P. semisulcatus) were investigated during a high shrimp recruitment peak lasting from January to June 2002, in four contiguous habitats within a non-estuarine mangrove bay at Saco da Inhaca, Inhaca Island, Southern Mozambique. A total of 14,976 specimens representing the five species were collected by means of a 1m beam-trawl fitted with a 2 mm mesh net with a cod-end. Every two weeks corresponding with spring tides on three consecutive nights, three trawls of 50 meters each were carried out each night in each habitat at 1.40–2.15 a.m. after the daily spring tide high water peak. Species distributions among the four habitats during the six months sampled showed significant differences in habitat preference (p>0.001). Fenneropenaeus indicus, M.stebbingi and P. japonicus dominated the catches in sand flats with densities of 27 ± 0.94, 18 ± 0.98 and 7 ± 0.76 shrimp /50 m2, respectively. Metapenaeus monoceros dominated catches in mudflats at a density of 21 ± 0.78 /50 m2 followed by M. stebbingi with 13±1.2 /50 m2. Penaeus semisulcatus was almost exclusively captured in seagrass meadows at densities of 8± 0.57 /50 m2. Only the Metapeneaus species showed a wide range of habitat utilization. The differences in habitat preference emphasises spatial partitioning between shrimps, reducing competition. Multiple linear regressions of juvenile shrimp densities for each species with water depth, salinity and temperature showed some significant effects. Fenneropenaeus indicus density showed a significant negative relationship with salinity and water depth and a positive relationship with temperature and turbidity. Metapenaeus stebbingi numbers showed a significant positive correlation with increasing salinity and temperature, but a negative one with depth. Penaeus japonicus numbers were significantly related to decreasing salinity and depth. Penaeus semisulcatus abundance was significantly related to decreasing temperature, and increasing turbidity and depth. More than 50% of the total shrimps captured had carapace length of ≤ 3 mm, substantiating the nursery function of the different habitats sampled in the study.
  • Primary carbon sources for juvenile penaeid shrimps in a mangrove-fringed Bay of Inhaca Island, Mozambique: a dual carbon and nitrogen isotope analysis.

    Macia, A. (WIOMSA, 2004)
    A study to estimate the relative importance of mangrove primary carbon and nitrogen sources to five commercial penaeid shrimps species was done at Saco da Inhaca, a non-estuarine mangrove-fringed bay on Inhaca Island, southern Mozambique. Carbon and nitrogen stable isotope ratios were determined in a variety of primary producers (mangroves, epiphytes, phytoplankton and seagrasses), sediments and in five penaeid shrimp species (Penaeus (Fenneropenaeus) indicus, P. japonicus, P. semisulcatus, Metapenaeus monoceros and M. stebbingi), collected within the bay in different habitats and during two different periods. The penaeid shrimps showed 13C values ranging from â 13 to â 19 â °, (average of â 15.6 ± 0.4 â °, n=19) which is highly enriched compared to the mean value for mangrove leaves (average - 27.6± 3.6 â °, n=3) which varied from â 20 to â 32â °. The results shows that some shrimps may derive their carbon either from detritus, plankton remains or from benthic organisms. Overall, the carbon isotopic signal shifted as the shrimps got bigger, suggesting a change of diet with growth. No significant differences were found between 15N isotope values among the shrimps studied, indicating that they may belong in the same trophic position (except P. semisulcatus, which occupies a higher level). There is some evidence that sampling period influences the carbon isotope ratios for sediment and shrimps.

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